In zinc electrowinning, zinc ions (Zn2+) are extracted from a zinc ore, and an anode and a cathode are dipped into a solution (hereinafter, an electrolyte) containing the extracted zinc ions and current flows between the anode and the cathode, thereby depositing high-purity zinc on the cathode. The electrolyte is an aqueous solution typically acidified with sulfuric acid, and therefore the main reaction on the anode is oxygen evolution. However, in addition to oxygen evolution, another reaction occurs on the anode. The reaction is oxidation of divalent manganese ions (Mn2+) contained in the electrolyte. The manganese ions are mingled into the electrolyte during the zinc ion extraction process. Specifically, in the zinc ion extraction process, the zinc ore is subjected to oxidizing roasting and then zinc ions are leached in the sulfuric acid solution, while in the roasting, some zinc contained in the zinc ore reacts with iron, thereby forming zinc ferrite. Zinc ferrite is a compound difficult to leach zinc ions from, and therefore in the course of leaching, manganese ore, manganese dioxide or potassium permanganate is added as an oxidant, thereby oxidizing and removing zinc ferrite. In this manner, zinc ferrite becomes removable, but the final sulfuric acid electrolyte having zinc ions extracted therefrom contains divalent manganese ions.
In the above-described zinc electrowinning, lead or a lead alloy is used as the anode, but for reasons such as a high oxygen evolution potential, high electric energy consumption required for oxygen evolution, and purity of zinc deposited on the cathode being reduced by lead ions dissolved from the anode, an insoluble electrode, which has a conductive substrate, such as titanium, coated with a catalytic layer containing noble metal or noble metal oxide, has been increasingly used as an anode which overcomes disadvantages as mentioned above. For example, Patent Document 1 discloses a copper electrowinning method which uses an insoluble electrode covered with an active coating containing iridium oxide. An insoluble electrode having titanium as a conductive substrate which is coated with an iridium oxide-containing catalytic layer, particularly, a catalytic layer comprising iridium oxide and tantalum oxide, has high catalytic properties and high durability with respect to oxygen evolution from an acidic aqueous solution and is used as an anode for oxygen evolution in electrogalvanizing or electrotining of steel or producing electrolytic copper foil. For example, in Patent Document 2, the present inventor discloses an oxygen evolution anode capable of inhibiting lead dioxide deposition on the anode during electrolysis as an insoluble oxygen evolution anode suitable for copper plating or electrolytic copper foil production. In recent years, application of such an insoluble anode is also under study in the field of metal electrowinning.
Also, in cobalt electrowinning, divalent cobalt ions (Co2+) are extracted from a cobalt-containing ore, and the anode and the cathode are dipped in a solution (hereinafter, an electrolyte) containing the extracted cobalt ions and current flows between the anode and the cathode, so that high-purity cobalt is deposited on the cathode. The solution is typically an acidic aqueous solution, and typical examples of the electrolyte include a chloride-based electrolyte obtained by dissolving divalent cobalt ions in an aqueous solution containing chloride ions typically acidified with hydrochloric acid and a sulfuric acid-based electrolyte obtained by dissolving divalent cobalt ions in an aqueous solution acidified with sulfuric acid. In cobalt electrowinning, the anode and the cathode are dipped in the electrolyte, a certain amount of cobalt is deposited on the cathode, and then the cathode is removed to recover cobalt. On the other hand, in the case where a chloride-based electrolyte is used, typically, the main reaction on the anode is chlorine evolution, and in the case where a sulfuric acid-based electrolyte is used, the main reaction is oxygen evolution. However, the main reaction on the anode may vary depending on the type of reaction to which the anode has catalytic properties, and both chlorine evolution and oxygen evolution may occur.
In the above-described cobalt electrowinning, a lead-based electrode, such as lead or a lead alloy, is mainly used as an anode, which is disadvantageous, for example, in that the anode reaction occurs at high potential, hence high electric energy consumption is required for the anode reaction, and lead ions dissolved from the anode reduce the purity of cobalt deposited on the cathode. Also, in the case where the lead-based electrode is used as an anode, chlorine or oxygen evolution, the main reaction on the anode, occurs, and simultaneously, a side reaction occurs in which divalent cobalt ions contained in the electrolyte are oxidized, so that cobalt oxyhydroxide (CoOOH) is evolved on the anode, and the divalent cobalt ions in the electrolyte that should be originally reduced on the cathode through the reaction are unnecessarily consumed on the anode. On the other hand, in such cobalt oxyhydroxide deposition, reaction of cobalt ions or cobalt oxyhydroxide with the material of the lead-based electrode also occurs at the same time, so that a compound is generated on the electrode, which is known to partially contribute to stabilization of the lead-based electrode, but because divalent cobalt ions to be deposited on the cathode are decreased due to divalent cobalt ions on the anode being consumed through reaction, the side reaction is principally unnecessary if the anode itself has high durability. As an anode for overcoming the above-described disadvantages related to the lead-based electrode, an insoluble electrode is under study which includes a conductive substrate, such as titanium, coated with a catalytic layer containing noble metal or noble metal oxide. For example, Non-Patent Document 1 describes cobalt electrowinning in which an insoluble electrode is used as an anode in a chloride-based electrolyte.